Why doesn't my Android Gmail update automatically?

Did it ever update automatically and now it stopped, or is it a new device?
Anyhow - settings => accounts => find your account and enable sync.If it used to work and just stopped doing so now =>>
Go to settings, applications, manage applications, all applications, gmail, CLEAR DATA. Reboot phone.
Some folks also found that using the power widget for the homescreen to toggle "master sync" on and off and on again sometimes gets things going again.

Disable sync, clear the cache and data of the gmail app, reboot, enable sync. Worked for me.